1. Field of the Invention
The present invention relates to elastomeric matrixes containing fibers and, more particularly, to electrically conductive gasket compositions.
2. Description of the Prior Art
A disadvantage of resilient plastic compositions which are suitable for use as gaskets is that they are transparent to electromagnetic interference (EMI). To remedy this problem, electrically conductive materials have been incorporated into the gasket compositions to shield against EMI. Representative prior art conductive materials comprise powders, flakes and fibers of carbon-graphite including metal coatings of aluminum, copper, steel and silver on the above.
A problem with many of the above materials is that their use within a plastic composition adversely affects the physical properties thereof. This is a particular problem in environmental gasket applications where the ability to seal against mechanical parts is a basic requirement. However, use of metallic powders in electrically conductive concentrations generally makes the plastic composition hard and ineffective as a seal. The addition of plasticizers or other modifiers may be helpful, but add to the cost and processing expense.
With metal coated fibers, the coating cracks during mixing, molding or use of the gasket. EMI shielding thereby drops precipitously. A particular problem with metal coated fibers of carbon-graphite with diameters less than 40 micron, is that, in addition to cracks in the coating, they are too flexible and fail to pierce the mating surface's microscopic layer of oxidation. As a result, conductivity between the gasket and associated mechanical parts is inhibited which, in turn, adversely affects shielding.
Also, the use of carbon-graphite and many other metallic materials create problems of galvanic corrosion. Such occur when the materials are electromechanically coupled, such as in a marine environment, with noncompatible mating surfaces. The products of corrosion form on the corresponding adjacent surfaces thereby inhibiting conductivity and effective shielding. The problem also occurs with chopped or cut metal coated graphite fibers. Here, the cut end of each fiber exposes the graphite. This provides a site for the above-described galvanic corrosion.